1. Field of the Invention
The present invention relates generally to wireless communication systems, and, in particular, to a wireless communication system adapted securely to perform channel sounding.
2. Description of the Related Art
In general, in the descriptions that follow, we will italicize the first occurrence of each special term of art which should be familiar to those skilled in the art of ultra-wideband (“MB”) communication systems. In addition, when we first introduce a term that we believe to be new or that we will use in a context that we believe to be new, we will hold the term and provide the definition that we intend to apply to that term. In addition, throughout this description, we will sometimes use the terms assert and negate when referring to the rendering of a signal, signal flag, status bit, or similar apparatus into its logically true or logically false state, respectively, and the term toggle to indicate the logical inversion of a signal from one logical state to the other. Alternatively, we may refer to the mutually exclusive Boolean states as logic_0 and logic_1. Of course, as is well known, consistent system operation can be obtained by reversing the logic sense of all such signals, such that signals described herein as logically true become logically false and vice versa. Furthermore, it is of no relevance in such systems which specific voltage levels are selected to represent each of the logic states.
By way of example, in an ultra-wideband (“UWB”) communication system, a series of special processing steps are performed by a UWB transmitter to prepare payload data for transmission via a packet-based UWB channel. Upon reception, a corresponding series of reversing steps are performed by a UWB receiver to recover the data payload. Details of both series of processing steps are fully described in IEEE Standards 802.15.4 (“802.15.4”) and 802.15.4a (“802.15.4a”), copies of which are submitted herewith and which are expressly incorporated herein in their entirety by reference. As is known, these Standards describe required functions of both the transmit and receive portions of the system, but specify implementation details only of the transmit portion of the system, leaving to implementers the choice of how to implement the receive portion.
One or more of us have developed certain improvements for use in UWB communication systems, which improvements are fully described in the following pending applications or issued patents, all of which are expressly incorporated herein in their entirety:
“A Method and Apparatus for Transmitting and Receiving Convolutionally Coded Data”, U.S. Pat. No. 7,636,397, issued 22 Dec. 2009;
“A Method and Apparatus for Generating Codewords”, U.S. Pat. No. 7,787,544, issued 31 Jul. 2010;
“A Method and Apparatus for Transmitting and Receiving Convolutionally Coded Data”, U.S. Pat. No. 8,358,709, issued 22 Jan. 2013; and
“Receiver for Use in an Ultra-Wideband Communication System”, U.S. Pat. No. 8,437,432, issued 7 May 2013;
“Convolution Code for Use in a Communication System”, U.S. Pat. No. 8,677,224, issued 18 Mar. 2014;
“Adaptive Ternary A/D Converter for Use in an Ultra-Wideband Communication System”, U.S. Pat. No. 8,436,758, issued 7 May 2013;
“Receiver for Use in an Ultra-Wideband Communication System”, U.S. Pat. No. 8,760,334, issued 24 Jun. 2014;
“Receiver for Use in an Ultra-Wideband Communication System”, U.S. Pat. No. 9,054,790, issued 9 Jun. 2015; and
“Adaptive Ternary A/D Converter for Use in an Ultra-Wideband Communication System”, U.S. Pat. No. 9,325,338, issued 26 Apr. 2016.
As is known, the 802.15.4a UWB PHY uses the following frame structure:
SyncSFDPHRDATA
The vulnerabilities here are:                1) if the start of the Sync is known in advance or detected by listening to the packet, the rest of the sync is entirely predictable.        2) The Sync is periodic, i.e., it repeats the same symbol again and again, so a version which is delayed by just one symbol looks like almost identical to the original with no delay.The code which is repeated in the Sync sequence is a so-called Ipatov code. (See earlier patents). Ipatov codes have the useful channel sounding property that they have perfect periodic auto-correlation (“PPAC”), i.e., if one of these codes is transmitted repeatedly hack to back, then correlating it with a copy of itself results in a Kronecker delta function (see, https://en.wikipedia.org/wiki/Kronecker_delta). The vulnerability identified above can be removed by changing the symbol at every symbol transition during the Sync sequence to one of the very large number of possible Ipatov codes, but this destroys the PPAC nature of these codes because they only have perfect auto-correlation if the same code is sent repeatedly back to back. However, if the code is changed each time a new symbol is sent, then the auto-correlation function has sidelobes which do not cancel out.        
Let us assume that we change the code for each symbol we send. As noted above, the sidelobes no longer cancel out, but those sidelobes do change with each code change. Whereas the peak of the auto-correlation is always equal to the number of pulses in the code (9 in these earlier examples), the sidelobes are always different. This means that we can actually get a good channel estimation by sending a long enough succession of different codes. Because the sidelobes are randomly positive or negative they eventually average out to zero. The problem with doing this is that it requires a much larger number of codes such that the sums of the sidelobes will eventually get small enough so as to be negligible, and hence a much longer estimation sequence than if we had used the same Ipatov code for every symbol. However, any code with good auto-correlation properties (i.e., having a good Golay Merit Factor) will do just as well.
We submit that what is needed is an improved method and apparatus for use in the receiver of a wireless communication system to perform channel sounding. In particular, we submit that such a method and apparatus should provide performance generally comparable to the best prior art techniques, but allow asymmetric delays to be used without significantly reducing accuracy.